This invention pertains to an improved insulated electrical conductor for use in dynamoelectric machines and also to dynamoelectric machine coils utilizing the insulated conductor of the present invention.
It has long been recognized that the insulated wire used in winding electric motor armatures should be capable of bending fairly sharply without breaking or cracking the insulation. For example, in winding the armature for an internal combustion engine starter motor a standard test has been employed to determine whether an insulated wire can withstand these bends. This simple test comprises wrapping the insulated wire around a mandrel having a diameter six times the diameter of the wire. If the insulation cracks or breaks while wrapping the insulated wire around the mandrel, the wire is not suitable for use in a starter motor armature.
Proposals have been made in the past which, in fact, do provide an insulated wire having insulation flexible enough for use in winding armatures. One such proposal has been a spirally wrapped paper insulated wire. This type of wire insulation owes its flexibility to the fact that each single turn of the spiral wrap is free to move as the wire is bent.
It has been found that when a spirally wrapped paper insulated wire is used in an armature winding, the wire might be exposed at some of the sharper bends due to the fact that the two turns adjacent the bend will tend to slip away from the bend. In order to overcome this problem, it has been necessary to overlap the spiral wrappings a great deal. By doing this, more paper is used in insulating the wire resulting in high cost.
Longitudinal wrappings of crepe paper have long been used for insulation of straight electrical wires. However, this type of insulated wire has not been considered suitable for armature windings because the paper insulation tears when the wire is bent in forming winding coils with end loops. Since armature coils are compactly fitted together in side-by-side relation, ruptured or weakened insulation is apt to result in a winding failure. While there are other materials such as woven fabrics which are stronger than paper, these materials have various disadvantages such as higher cost, greater thicknesses, difficulty of application.
In accordance with the present invention, the properties of conductors insulated with a single ply longitudinal wrapping of a non-woven, non-crepe sheet insulation material are significantly improved by use of an insulation wrapping of a material which is stretchable to an extent of at least 12% in a longitudinal direction axially parallel with the conductor and to an extent of at least 8% in a transverse direction without rupturing. A rag paper having such stretching characteristics is particularly useful for the insulation of solid wires having a cross-sectional area of at least 0.0025 square inches, particularly round wires of a size in the range of No. 15 A.W.G to No. 6 A.W.G. A tape of this material is longitudinally wrapped in tubular form around the conductor with the edges of the sheet overlapping and adhesively bonded to each other. The inside surface of the tape is in contact with the conductor and is free throughout its length for longitudinal movement on the conductor. Such improved insulated conductors successfully pass the above-described mandrel test and are quite suitable for use as motor armature coils having sharply bent end loops.
Thus, it is an object of the present invention to provide an electrical conductor insulated with a longitudinally wrapped tape which is useful in coils for electric motor armatures and the like.